But have you ever wondered what kind of message this wish carries? You’re basically wishing for this someone to consume their food with pleasure. It would seem that this is quite innocent and is indeed a sign of civility and good manners. This is what's done "in velt", as they say. But us being Jews and all, even this minor nuance is different!
What do Jews do when they see others eating? We say “tzu gezunt !” (to health) and we sincerely mean it, too!
To understand the difference between these two wishes, one must first understand why we actually eat. For fun? Or to maintain the vitality of the body?
To those familiar with the teachings of Chassidus, it is quite obvious that getting pleasure from eating and drinking can lead to pretty disastrous consequences. And we're not just talking physical consequences. It's more about spiritual realities. But since man consists of body and soul, spiritual issues may entail physical disorders and diseases.
Our animal soul loves to get pleasure from material things: pocessions, food, and whatever else. Therefore, whenever we pander to it, we strengthen the animal soul whereas that extra strength could be directed to support the divine soul in the Jew instead. As a result, the animal soul gains power and can attempt to dictate terms to our body, thereby knocking it off the path of Torah and the commandments.
Again – it’s not a rejection of tasty food in total, indeed, it’s part of Shabbos and holiday enjoyment. Rather, it’s control over the thoughts with which we absorb it. If these thoughts are about what we eat and drink in order to maintain the vitality of our body, which is an important way to serve the A-mighty, then the life-giving energy extracted from the food we eat will go to where it should - to the divine soul.
It is very difficult work - constantly controling our feelings, but each of us has enough hidden forces for it.
It may happen that one may not be aware of this concept but when we wish them a hearty “tzu gezunt’ rather than a bland "bon appetit", we help them redirect their thinking about what’s primary and what’s secondary. In addition, our positive words and wishes have a hidden force which also allows us to strengthen our divine soul in general.